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Khandro, the cricket ball will only move at the same speed as the car when a force (acceleration) is applied to it. The Earth's gravity has absolutely nothing to do with it. If the car was somewhere in interstellar space the cricket ball would stay where it was when the car was accelerated until it impacted some part of the car.
jomifl; //The Earth's gravity has absolutely nothing to do with it.//
I'm not sure if we are talking at cross purposes, but gravity has a lot to do with what I'm saying which is;
If you place a cricket ball on the floor of say a stationary estate car near the front, the ball will stay where you place it - natch, (held in place by the pressure of gravity). If you then accelerate away the ball will roll to the back of the car until it is stopped at the tailgate. What is happening is that the ball is actually trying to remain in its position in relation to the Earth and it is the car's forward motion creating it to roll.
Do you agree?
I'm not sure if we are talking at cross purposes, but gravity has a lot to do with what I'm saying which is;
If you place a cricket ball on the floor of say a stationary estate car near the front, the ball will stay where you place it - natch, (held in place by the pressure of gravity). If you then accelerate away the ball will roll to the back of the car until it is stopped at the tailgate. What is happening is that the ball is actually trying to remain in its position in relation to the Earth and it is the car's forward motion creating it to roll.
Do you agree?
No. The ball is not held there by Earth's gravity. The ball is not 'trying' to remain anywhere. This might help :-
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Khandro - it is very obvious that you have not studied applied maths at A Level. Gravity is absolutely irrelevant as it is acting at right-angles to the movement. The ball will stay exactly where it is in space until a force acts on it. The car moves because some force acts on it to move it forwards; the ball stays exactly where it is in space ie; the car moves under it. When the car has moved far enough the car hits the ball and provides a force which causes it to move in the same direction as the car.
bhg; That is exactly what I am saying - //The ball will stay exactly where it is in space until a force acts on it.// it stays where it is because of gravity until the car moves from under it, causing it to roll in order to stay "where it is".
It is the car which initially moves, and the ball attempts to remain where it is, producing its movement within the car. The car and the ball are both moving in relation to the Earths surface, but until the ball's movement is arrested by the tailgate, the car and the ball are moving at different speeds.
It is the car which initially moves, and the ball attempts to remain where it is, producing its movement within the car. The car and the ball are both moving in relation to the Earths surface, but until the ball's movement is arrested by the tailgate, the car and the ball are moving at different speeds.
Khandro: // are you saying it will hit the same spot on the floor? //
No, it would hit a different spot, but not because of a loss of forward momentum. It's because of the curvature of the earth. While stationary, the train is rotating with the earth, hence floor and ceiling have different velocities, as mibn2cweus said. Whilst moving the train has some additional rotation (or less if it's moving against the earth's rotation), hence the floor-ceiling velocity differential would be different and the landing spot would be different. If we lived on a flat, non-rotating earth, your dropped ball bearing *would* land on the same spot irrespective of the velocity of the train — only acceleration would affect where it landed, not velocity.
No, it would hit a different spot, but not because of a loss of forward momentum. It's because of the curvature of the earth. While stationary, the train is rotating with the earth, hence floor and ceiling have different velocities, as mibn2cweus said. Whilst moving the train has some additional rotation (or less if it's moving against the earth's rotation), hence the floor-ceiling velocity differential would be different and the landing spot would be different. If we lived on a flat, non-rotating earth, your dropped ball bearing *would* land on the same spot irrespective of the velocity of the train — only acceleration would affect where it landed, not velocity.
"it stays where it is because of gravity until the car moves from under it, causing it to roll in order to stay "where it is". "
Right idea but gravity has nothing to do with it. You would observe the same behaviour in deep space, the ball tending to roll back relative to the car. Not gravity, but inertia. Newton's First Law is basically what matters here: the ball isn't the one doing the accelerating, and if it's smooth enough then friction between it and the car won't be enough to couple the systems, so it stays still (relative to the car). The force of gravity merely serves to pull it down to the floor, but once the ball is on the floor then a reaction force balances against gravity (Newton's Third Law) and removes it from further consideration of the motion of the ball (Newton's second law, in that the sum of external forces must be non-zero in order for an object to accelerate; the sum of forces in the vertical direction on the ball is zero).
So no, it is not held in place "because of gravity", or anything like that.
Right idea but gravity has nothing to do with it. You would observe the same behaviour in deep space, the ball tending to roll back relative to the car. Not gravity, but inertia. Newton's First Law is basically what matters here: the ball isn't the one doing the accelerating, and if it's smooth enough then friction between it and the car won't be enough to couple the systems, so it stays still (relative to the car). The force of gravity merely serves to pull it down to the floor, but once the ball is on the floor then a reaction force balances against gravity (Newton's Third Law) and removes it from further consideration of the motion of the ball (Newton's second law, in that the sum of external forces must be non-zero in order for an object to accelerate; the sum of forces in the vertical direction on the ball is zero).
So no, it is not held in place "because of gravity", or anything like that.
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